A transmission is used to transmit power from an engine to a drive mechanism. The transmission uses the principle of mechanical advantage to convert the rotational speed, direction, and torque of a driving element into a different rotational speed, direction, and torque of a driven element. Most transmissions use a combination of gears in differing ratios to achieve this speed-torque conversion.
Vehicle transmissions often include more than one set of gear ratios (typically called “gears”) to allow the vehicle to operate in a variety of conditions. When the vehicle is at rest or travelling at a low speed, a gear ratio may be selected to deliver relatively high torque from the engine to the driveline. When the vehicle is travelling at higher speeds, a different ratio may be used to deliver higher rotational speeds at lower torque to the driveline. Gear ratio may be selected to optimize the delivery of power to the driveline having regard to the characteristics of the engine, and in particular, to the engine's delivery of power as a function of the engine's rotational speed. Changing the gear ratio of a transmission is commonly known as shifting or changing gears, and typically requires a brief decoupling of the engine from the driveline using a clutch arrangement.
A typical vehicle transmission as exemplified in FIG. 1 may include an input shaft 101 which is driven by the engine through a clutching arrangement, and an output shaft 102 which may drive the driveline when a gear is selected. The input shaft 101 typically passes through a number of input gears 110. The output shaft 102 in this arrangement may pass through a number of corresponding output gears 109 which mesh with the input gears 110, each pair of meshed gears being a gear set. In each gear set, one of the gears is not directly affixed to the input or output shaft, and may spin independently of the shaft when not engaged, and the other is affixed to the input or output shaft. Adjacent to the each free spinning gear 111, a sliding gear 108 may be mounted on the shaft passing through that free spinning gear 111. Each sliding gear 108 may slide along the length of the shaft, but otherwise engage the shaft so that it rotates along with the shaft. Each free spinning gear 111 may have dog teeth which engage with the adjacent sliding gear 108 when the sliding gear 108 is slid along the shaft. The sliding gear 108 engages and rotates the free spinning gear 111 with the corresponding shaft, thereby selecting a gear. Once engaged, the output shaft 102 is driven by the input shaft 101 in a ratio determined by the selected gear.
Although this is a common implementation of a vehicle transmission, there are many variations which achieve the same function in a similar manner.
Some vehicles use a sequential transmission, which is a transmission having at least two sets of gears which must be selected in a predetermined order during shifting. If a vehicle has three gears, the sequential transmission cannot be shifted from any one gear set to any other gear set. It must be shifted in an order which is determined by the configuration of the gear changing mechanism.
In a sequential transmission implemented on the typical vehicle transmission described above as exemplified in FIG. 1, the sliding gears 108, are moved by selector forks 106 that slidably engage a selector fork shaft 107 which is aligned parallel to a selector drum 104. Typically, there are grooves 105, wedges or ridges on a selector drum 104 which engage the selector forks 106 and convert the rotation of the selector drum 104 into lateral movement of the selector forks 106 along the selector fork shaft 107 in a direction parallel to the input shaft 101 and output shaft 102, thereby moving the sliding gears 108 along the input shaft 101 or output shaft 102. The use of a selector drum 104 to select gears in this arrangement forces the operator to shift gears in order. Gear dog teeth 112 located on the side of sliding gears 108 are used to engage gear dog windows 113 located on the common side of the free spinning gear 111. This engagement effectively locks the free spinning gear 111 to the shaft running through its hub and permits torque to be transmitted from the input shaft 101 to the output shaft 102 through the free spinning gear 111 and its meshing gear 119.
Sequential transmissions are preferred in certain applications over other types of transmissions because of the relative simplicity of the apparatus. A typical sequential transmission has fewer moving parts and is generally more reliable than a comparable fully manual non-sequential transmission. They can often be made smaller and lighter than other comparable designs, and can be faster to complete gear shifts. They are often employed in automotive racing and motorcycle applications for these reasons.
Many sequential transmissions are driven manually by the operator using hand or foot levers that may rotate the selector drum 104 through a ratcheting arrangement 114. This allows the operator to rotate the selector drum 104 enough to cause the shift, but helps prevent the operator from rotating the selector shaft too far. When implemented on a motorcycle, the sequential transmission may include an indexer arrangement, such as a cam indexer 116. The indexing arrangement may include a cam sprocket 115 connected to the selector drum 104 in combination with a pawl or cam follower 118 that engages depressions in the cam sprocket 115 as the selector drum 104 is rotated. The cam follower 118 may have a wheel 117 at one end which may roll along the cam sprocket 115, and may be biased so that the wheel 117 maintains contact with the cam sprocket 115. When the cam follower 118 is seated in a depression, the selector drum 104 has been rotated to a position where a gear is engaged. By applying force to a shift lever attached to the ratcheting arrangement 114, the operator may rotate the selector drum 104 if the force is sufficient to unseat the cam follower 118 from the depression and overcome friction forces. As the selector drum 104 rotates, the cam follower 118 will move into an adjacent depression on the cam sprocket 115.